Science Musings Blog

Thursday, January 23, 2014

Back of the envelope

The other day my pal Dwight and I were sitting on the "treehouse" deck at Houseman's bar, nursing cold Kaliks and looking out at the sea. I launched into some explanation or other about water molecules.

"How big is a water molecule?" asked Dwight.

How to answer?

I remembered a rough back-of-the-envelope calculation I did here back in June 2011, during the Panama voyage. "There are about as many molecules in a thimbleful of water as there are thimblefuls of water in all the oceans of the Earth," I said.

I didn't tell Dwight that I had done the calculation; he just supposed it was something an ex-physicist might know. I love doing this sort of order-of-magnitude calculation. I used to occasionally assign them to my students. You'll need to look up the mass of a proton, the radius of the Earth, and the average depth of the oceans, all readily available with Google. The rest is just high-school math and chem. Those of you with some science/math background can try it.

Anyway, you must admit, it's a fairly mind-blowing comparison. As we sat there looking out at the sea stretching away to the far horizon and imagining all the thimblefuls of water, suddenly the teeming world of molecules came into better perspective.